Chemical looping technology is a promising technology intended to achieve gasification or combustion of solid fuels replacing direct oxygen feed by using an oxygen carrier.
Chemical looping is a process by which combustion or gasification of a carbon-based fuel occurs in two steps. In a first step, the oxygen carrier delivers oxygen in a fuel reactor where the oxygen carrier is reduced by the solid fuel and is then transferred to an air reactor. Fuel particles, ash and reduced oxygen carrier are produced in the fuel reactor. In a second step, the reduced oxygen carrier is oxidized by air in the air reactor. In the chemical looping process, the gas resulting from the combustion or gasification of the fuel in the fuel reactor is then free or nearly free from nitrogen. The oxygen carrier can typically comprise a metal oxide or other oxygen rich compounds.
Most of chemical looping technologies use the fluidized beds technology for the fuel reactor to benefit from the high residence time as well as from the good mixing associated with fluidized beds. Fluidized beds can be bubbling fluidized beds or circulating fluidized beds.
Typical crushed coal that is introduced into conventional systems for converting fuel material has a mean diameter of 2 mm and the top size of the coal can reach 20 mm. The biggest particles that do not end as fly ash have hence to be removed as bottom ash.
The current ratio between fly ash and bottom ash is 60/40 and tends to go down to 40/60. A dedicated bottom ash extraction system has to be designed to remove approximately half of the ash introduced via the fuel material into the system.
Thus, a major challenge is to separate the fuel particles and the ash from the oxygen carrier before sending the oxygen carrier to the air reactor for oxidation and regeneration.
The conventional chemical looping systems for converting fuel material comprise a carbon separator, as mentioned for instance in document FR 2 850 156. The carbon separator, which is also called “carbon stripper”, is placed between the air reactor and the fuel reactor.
The conventional chemical looping systems also comprise an ash separator placed at the bottom of the fuel and/or air reactor in order to drain ash from the system. This extraction is typically located at the very bottom of the reactor. Several designs exist with different locations for the extraction hole. The hole can be located in the middle of the grate of the reactor by removing some nozzle which leads to a vertical extraction. The hole can be located on a side wall of the reactor with a lateral extraction with a flow that is controlled by a cone valve.
These systems suffer the shortcoming that the ash separator involves a heavy and complex separator system at the bottom or nearly the bottom of the fuel and/or air reactor.
Moreover, the solids mixture at the bottom of fuel and air reactors typically comprises about 10% ash. If 1 kg/s ash is to be extracted, this would mean that 10 kg/s of the mixture containing 1 kg/s ash will be extracted. Thus, the solid quantity that would be needed to treat is ten times higher than the really needed solid quantity.